Gas-detecting device

ABSTRACT

A gas-detecting device having a gas responsive semiconductor, a pair of electrodes in contact with the semiconductor, means for heating the semiconductor to stabilize its resistance at a predetermined value, time delay switching means and an alarm circuit and an impedance circuit interconnected with said electrodes and said switching means so that the impedance circuit will be connected with the semiconductor electrodes during the heating period and upon stabilization of the semiconductor the switching means will automatically and effectively disconnect the impedance circuit and substitute the alarm circuit.

United States Patent [72] Inventor Naoyoshi Taguchi 1-2 IkedauemachiNagata-ku, Kobe, Japan [21] Appl. No. 54,743

[22] Filed July 14, 1970 [45] Patented Dec. 28, 1971 [54] GAS-DETECTINGDEVICE 6 Claims, 4 Drawing Figs.

[52] U.S. Cl 340/237, 23/254 E [51] Int. Cl G08b 21/00, G0 1 n 31/06[50] Field of Search 340/237 R;

23/254 R, 254 E, 232 R, 232 B; 73/23, 25-27 [56] References Cited UNITEDSTATES PATENTS 2,533,339 12/1950 Willenborg 340/237 2,666,105 1/1954Menozzi et al 340/237 X 3,479,257 11/1969 Shaver 23/232 X FOREIGNPATENTS 754,087 8/1956 England 340/237 Primary Examiner.l0hn W. CaldwellAssistant ExaminerDaniel Myer Attorney-Eugene E. Geoffrey, Jr".

ABSTRACT: A gas-detecting device having a gas responsive semiconductor,a pair of electrodes in contact with the semiconductor, means forheating the semiconductor to stabilize its resistance at a predeterminedvalue, time delay switching means and an alarm circuit and an impedancecircuit interconnected with said electrodes and said switching means sothat the impedance circuit will be connected with the semiconductorelectrodes during the heating period and upon stabilization of thesemiconductor the switching means will automatically and effectivelydisconnect the impedance circuit and substitute the alarm circuit.

GAS-DETECTING DEVICE This invention relates to a detector for gas, smokeand other air-contaminating vapors and more specifically to a detectingdevice utilizing a metal oxide semiconductor element which varies inimpedance when exposed to reducing gases such as hydrogen, carbonmonoxide, alcohol vapor, gasoline vapor or smoke and embodies means forpreventing the production of an erroneous alarm during initial periodsof operation when the resistance of the metal oxide semiconductor isunstable.

Detecting devices for gas, smoke, and other similar air contaminantsutilizing metal oxide semiconductor elements must be heated to apredetermined temperature in order to increase their sensitivity. Whilethe desired operating temperature depends on various characteristics ofthe semiconductor element such as its geometry, and the desireddetecting sensitivity, it has been determined that a temperature in therange of 170 to 230 C. is generally preferable.

Assuming that the metal oxide semiconductor element of the detector hasa geometry such that the interelectrode resistance is of the order to50,000 ohms when heated to a temperature in the range of 170 to 230 C.and when the ambient air does not contain any contaminant such as gas orsmoke, such semiconductor will vary in resistance from an exceedinglyhigh value of the order of hundreds of thousands of ohms to values aslow as 5,000 ohms during a period of approximately tour minutes requiredto bring the semiconductor up to its stable operating temperature. Underthese conditions the semiconductor detector will produce an alarm duringthe initial heating period even though smoke or gas does not contaminatethe ambient air.

This invention overcomes the aforementioned disadvantages and utilizesswitching means associated with the alarm circuit to prevent thegeneration of an alarm until after the semiconductor has attained apredetermined temperature. Semiconductor elements do not reach a stablestate until after it has been heated to a temperature that will permit apredetermined current to flow between the electrodes. In priorgas-detecting devices thermisters have been inserted with the alarmcircuit to delay initiation of the alarm circuit. While this procedurehas certain advantages, it does not provide the desired reliability anddependability for an alarm device.

Accordingly one object of the invention resides in the provision of anovel and improved circuit for gas detectors having metal oxidesemiconductor elements which will provide stable operation throughoutthe entire temperature range. This may be obtained in accordance withthe invention by utilizing an auxiliary circuit having an impedancesubstantially equivalent to he capacitance of the alarm circuit andmeans for interconnecting the auxiliary circuit with the semiconductorelement until the latter reaches a predetermined temperature andoperation is stabilized. After the semiconductor element has becomestabilized, the alarm circuit is automatically substituted for theauxiliary circuit to place the detector in an operational condition.

The gas detector in accordance with the invention includes a metal oxidesemiconductor element having a resistance which varies materially whenexposed to a reducing has or smoke and a pair of electrodes in contactwith the semiconductor element to provide for the flow of currentthrough the semiconductor. The device further includes a heater forheating the semiconductor, an alarm circuit, an auxiliary impedancecircuit having substantially the same impedance as the alarm circuit anda delayed switching means for connecting the auxiliary circuit to one ofthe electrodes for a predetermined time period after energizing theheating element and at the conclusion of the time period at which thesemiconductor element reaches a predetennined temperature or itsso-called steady state, disconnecting the auxiliary circuit andsubstituting the alarm circuit therefor.

The above and other objects and advantages of the invention will becomemore apparent from the following description and accompanying drawingsforming a part of the application.

In the drawings:

FIG. 1 is a graph showing the change in resistance of a metal oxidesemiconductor element during the heating period;

FIG. 2 is a cross-sectional view of one embodiment of a gasdetectingdevice in accordance with the invention together with a schematiccircuit diagram;

FIG. 3 is a plan view in partial section of a gas-detecting device ofFIG. 2; and

FIG. 4 is a cross-sectional view of a modified embodiment of thedetector and circuit shown in FIG. 2.

The gas-detecting device in accordance with the invention utilizes ametal oxide semiconductor element that is heated to he temperature ofthe order of to 230 C. to stabilize its detecting sensitivity. It hasbeen found that this temperature range affords a smooth and continuouschange of resistance of the semiconductor element in response to changesin concentration of gases and smoke in the ambient air and thus issuitable for practical applications. Assuming that the metal oxidesemiconductor element has a geometry such that its interelectroderesistance is of the order of 50,000 ohms when heated to a temperatureof the order of 170 to 230 C. Such a semiconductor element will overcomeresistance variations during the warmup period as illustrated in FIG. 1.The initial resistance of the semiconductor prior to heating is of theorder of hundreds of thousands of ohms and may be deemed to be aninsulator at normal temperatures. When the semiconductor is heated, theresistance initially drops to as low as 5,000 ohms after the firstthirty seconds, and this condition continues for approximately2%minutes. Thereafter the resistance gradually increases until itreaches the inherent value of the material, namely, approxi mately50,000 ohms after a period of 4 minutes.

The change in resistance of the semiconductor element during the initialheating period as illustrated in FIG. 1 is believed to result from thefact that the semiconductor absorbs water and gases while it is cold andthen during the heating step, the water and gas react with thesemiconductor material to reduce its resistance. After a predeterminedperiod of heating, the water and has are expelled from the element andthe resistance then approaches the inherent resistance of the materialwhich in the present case is of the order of 50,000 ohms.

When a reducing gas or smoke contacts a heated semiconductor element,its resistance will vary materially. With semiconductors of thereduction type, the resistance drops from approximately 50,000 ohms toapproximately 10,000 ohms as shown in FIG. 1 after the time t,. Withsemiconductors of the oxidation type, the resistance increases toapproximately 100,000 ohms as shown in FIG. I.

Reduction-type metal oxide semiconductors will generate an alarm whenthe resistance falls below 50,000 ohms, and accordingly, during theheating time period of 30 seconds to 3 minutes, the semiconductorresistance will drop sufficiently to create erroneous alarms. In thecase of an oxidation-type semiconductor material, the erroneous alarmwould be generated within the first 30 seconds of the heating period.

One embodiment of a structure in accordance with the invention isillustrated in FIGS. 2 and 3. This structure includes a gas-detectingelement generally denoted by the numeral 2 and a surrounding metal capgenerally denoted by the numeral 4. The gas-detecting element 2 includesa cup-shaped electrode 6 open at the bottom end and having a number ofopenings therein to provide for the flow of air. A metal oxidesemiconductor material 8 fills the cup-shaped electrode 6 which in thecase of a reduction type semiconductor would be Sn(),. A secondelectrode 10 which is in the form of a flat or cylindrical rod isembedded in the semiconductor material 8 and extends downwardly into aninsulating plate 12 which is secured and closes the bottom end of theelectrode 6. The

electrode 10 may be formed of a nickel-chromium alloy and furtherincludes a heating wire 14 which is wound about and insulated from theelectrode 10. One end of the heating wire 14 is connected to a conductor16 which extends downwardly through the insulating plate 12. The otherend of the heating wire is connected to a conductor 20 which alsoextends downwardly through the insulating plate 12. The junction 18 ofthe heating wire 14 and the conductor 20 is also electrically connectedto the electrode so that the conductor also serves as a connection toone of the electrodes of the gas-detecting element.

The cap 4 has a plurality of holes 22 extending therethrough to permitsmoke to enter the cap. The cap 4 is also electrically connected to theelectrode 6 by conductors 24 and is fixed to an insulating plate 26.

A bimetallic switch 28 includes a bimetallic element 30 which iselectrically and mechanically coupled to the top of the cap 4. Theswitch 28 has one contact 32 connected through an alarm device 34 andthen to the terminal 52 of a secondary winding 46 of the transformer 40.The primary winding 42 is connected by means of a plug 44 to a suitablesource of alternating current. A second contact 36 is connected througha resistor 38 having an impedance substantially equivalent to the alarmcircuit 34 and thence to the terminal 52 of winding 46. The conductor 16of the heater winding 14 is connected to the terminal 48 of thesecondary winding 46 while the conductor 20 is connected to the terminal50 of the winding 46.

With the aforementioned structure that portion of the secondary windingbetween the terminals 48 and 50 may provide approximately 1.5 volts forsupplying energy to the heater 14. The portion of the secondary winding46 between the terminals 50 and 52 provides approximately 100 volts forproducing a current flow through either the resistor 38 or the alarmcircuit 34 as the case may be and thence through the semiconductor 8.

When the metal oxide semiconductor element 8 is not heated, the contactcarried by the bimetallic element 30 will be engaged with the contact36. When energy is supplied to the transformer 40, the bimetallic switch28 will be in the position as illustrated in solid lines placing theresistor 38 in circuit with the semiconductor 8. Under these conditionsan erroneous alarm cannot occur. After a period of time which permitsthe semiconductor 8 to attain a predetermined temperature and thusmaintain a stable resistance, the bimetallic switch 30 will be operatedand moved to the dotted line position shown in FIG. 2 to disconnect theresistor 38 and connect the alarm circuit 34 with the electrodes 6 and10. In the case of a reduction type semiconductor 8 the bimetallicswitch will prevent an erroneous alarm even though the semiconductorresistance may drop to as low as 5,000 ohms. It will be observed thatthe temperature of the outer cap 4 is spaced from the electrode 6 andthus its temperature will increase very slowly as it is not in intimatecontact with the gas-detecting element 2 and also has a fairly largethermal capacity. Thus the bimetallic switch will operate only after thesemiconductor element 8 reaches a stable value of the order of 50,000ohms. Therefore, by selecting a heater 14 of appropriate heatingcapacity and utilizing a cap 4 of predetermined size, a delay ofapproximately 4 minutes can be obtained before the bimetallic switch 28functions to switch the system from the resistor 38 to the alarm system34 at which time the semiconductor 8 will be sufficiently stabil- Whengas or smoke is not present in the ambient air or the concentration isbelow a specific value, the interelectrode resistance of thesemiconductor 8 when at its stable operating temperature will be about50,000 ohms and the current flowing through the alarm circuit 34 will betoo small to activate the alarm. However, if the air becomescontaminated with a gas such as hydrogen, carbon monoxide, or a vapor ofan organic fuel such as alcohol or gasoline or if the air becomescontaminated with smoke at a specific concentration, then such gas,vapor or smoke will pass through the porous electrode 6 and penetratethe metal oxide semiconductor 8. This will cause the resistance in thecase of a reduction type semiconductor to abruptly decrease as shown bythe curve in FIG. 1 following the time 1,. The decrease in resistancewill increase the current flowing through the alarm 34 and thus activateit. When the concentration of gas, vapor or smoke in the ambient airfalls below a specific concentration, the resistance of thesemiconductor will return to its normal or original value within a timeof several seconds to several minutes.

Should the resistance of the metal oxide semiconductor 8 require morethan 4 minutes to become stabilized then a bimetallic switch having agreater time delay would be utilized or in the alternative the size ofthe cap 4 may be increased to introduce additional delay. In thealternative should the semiconductor element become stabilized in lessthan 4 minutes, a more sensitive bimetallic switch may be employed. Inaddition to the utilization of SnO,as the semiconductor 8, suchmaterials as ZnO, Fe,0,, TiO,, V,O MnO,, W0 Th0,, M00 CdO, and PbCrO mayalso be utilized.

A modified embodiment of the invention is illustrated in FIG. 4, andthis embodiment of the invention utilizes a semiconductor element inplace of the bimetallic switch 28 of the embodiment shown in FIG. 2Since the detecting element of FIG. 2 is identical to the detectingelement of FIG. 4, only the switch operation will be discussed.

In the embodiment of FIG. 4, a temperature-responsive resistor 62 isafiixed to the cap 4 and has a resistance characteristic which decreaseswith an increase in temperature. A second temperature responsiveresistor 66 is also fixed to the cap 4, and its resistance increaseswith an increase in temperature. For example, a therrnister or acritesistor may be utilized as the resistor having a negativetemperature characteristic while a posistor or semistor may be utilizedas the resistor having positive temperature characteristics.

When the heater 14 is first energized and the cap 4 has not becomeheated, the resistance of the resistor 66 will be very much below thatof the resistor 62. Accordingly, during the initial heating period whenthe resistance of the semiconductor 8 is unstable, current will flowthrough the resistor 66 and the resistor 38 as in the case of theembodiment illustrated in FIG. 2. At the same time insufficient currentwill flow through the alarm 34 to activate it. Thus an erroneous alarmcannot be produced during the initial heating period. When the resistance of the semiconductor 8 is stabilized and the temperature of thecap 4 is raised to a predetermined level after a specific time period,the resistor 66 will greatly increase in value while the resistor 62will materially decrease in value. This action affectively connects thealarm circuit 34 to the detecting device and disconnects the resistor38. Under this condition the device is operable to detect contaminationin the ambient air.

The embodiments of the invention as described above afford a number ofadvantages. For instance through the utilization of an impedance circuithaving the same impedance as the alarm circuit and by interconnectingthe impedance circuit with the semiconductor during the heating period,stabilization of the semiconductor 8 is accomplished under normaloperating conditions so that the substitution of the semiconductor 8 isaccomplished under normal operating conditions so that the substitutionof the alarm circuit for the impedance circuit will not adversely affectthe detector. Furthermore, a smooth and rapid transfer from an unstablestate during the initial heating period to the normal operating state isaccomplished without any chance of the production of an erroneous alarm.Moreover, since the entire operation is automatic, human errors that mayoccur in preparing the detector for operation will be eliminated.

While the time delay switch in each embodiment of the invention has beendescribed as a thermal switch, it is evident that a timing switch whichfunctions after a predetermined time period corresponding to the timerequired for stabilization of the semiconductor element 8 may also beutilized.

While only certain embodiments of the invention have been illustratedand described, it is apparent that alterations, modifications andchanges may be made without departing from the true scope and spiritthereof as defined by the appended claims.

What is claimed is:

l. A gas-detecting device comprising a metal oxide semiconductor elementhaving a resistance characteristic which changes in the presence of acontaminating gas in the ambient air, a pair of electrodes in contactwith said semiconductor including means for producing a current flowthrough said semiconductor, means for heating said semiconductor to apredetermined temperature, an alarm circuit having a predeterminedimpedance, an impedance, circuit having the same impedance as the alarmcircuit and time delayed switching means interconnected with said alarmcircuit, said impedance circuit and said electrodes, said switchingmeans nonnally connecting said impedance circuit with said electrodesand maintaining the last said connection for a predetermined time afterenergizing said heating means and then automatically disconnecting saidimpedance circuit and substituting said alarm circuit whereby said alarmcircuit will be activated upon a predetermined resistance change of saidsemiconductor.

2. A gas-detecting device according to claim 1 wherein said time delayswitching means includes a temperature-sensing element responsive to thetemperature of said semiconductor element and a double throw switchactuated by said temperature changing element and connections betweenthe last said switch and said impedance circuit and said alarm circuit.

3. A gas-detecting device according to claim 2 wherein said time delayedswitching means includes a bimetallic element.

4. A gas-detecting devices according to claim 2 wherein saidsemiconductor element is enclosed within a perforated metal cap and saidtemperature-sensing element is thermally coupled to said cap.

5. A gas-detecting device according to claim 1 wherein said time delayswitching means includes a first resistor having a positive temperaturecharacteristic, a second resistor having a negative temperaturecharacteristic, means thermally coupling said resistors to saidsemiconductor element and connections between said first resistor andsaid impedance circuit and between said second resistor and said alarmcircuit.

6. A gas-detecting element according to claim 5 wherein said thermalcoupling means comprises a metal cap enclosing said semiconductorelement.

1. A gas-detecting device comprising a metal oxide semiconductor elementhaving a resistance characteristic which changes in the presence of acontaminating gas in the ambient air, a pair of electrodes in contactwith said semiconductor including means for producing a current flowthrough said semiconductor, means for heating said semiconductor to apredetermined temperature, an alarm circuit having a predeterminedimpedance, an impedance circuit having the same impedance as the alarmcircuit and time delayed switching means interconnected with said alarmcircuit, said impedance circuit and said electrodes, said switchingmeans normally connecting said impedance circuit with said electrodesand maintaining the last said connection for a predetermined time afterenergizing said heating means and then automatically disconnecting saidimpedance circuit and substituting said alarm circuit whereby said alarmcircuit will be activated upon a predetermined resistance change of saidsemiconductor.
 2. A gas-detecting device according to claim 1 whereinsaid time delay switching means includes a temperature-sensing elementresponsive to the temperature of said semiconductor element and a doublethrow switch actuated by said temperature changing element andconnections between the last said switch and said impedance circuit andsaid alarm circuit.
 3. A gas-detecting device according to claim 2wherein said time delayed switching means includes a bimetallic element.4. A gas-detecting devices according to claim 2 wherein saidsemiconductor element is enclosed within a perforated metal cap and saidtemperature-sensing element is thermally coupled to said cap.
 5. Agas-detecting device according to claim 1 wherein said time delayswitching means includes a first resistor having a positive temperaturecharacteristic, a second resistor having a negative temperaturecharacteristic, means thermally coupling said resistors to saidsemiconductor element and connections between said first resistor andsaid impedance circuit and between said second resistor and said alarmcircuit.
 6. A gas-detecting element according to claim 5 wherein saidthermal coupling means comprises a metal cap enclosing saidsemiconductor element.